One of the trends in present electronic systems is the development of high speed digital circuits with a relatively large number of circuit interconnects between circuit boards. In addition to such higher operating speeds, increased circuit density and faster signal rise times are placing greater demands on circuit designers. Signal transmission in such faster, higher speed digital processing systems for computer applications and the like are thus becoming increasingly complex. The overall efficiency of signal transmission is affected by each element of the system, for example, the integrated circuit, printed circuit boards, electrical connectors, as well as the interfaces between each element. Maintaining the efficiency and integrity of a signal from a motherboard to a daughterboard in a high speed environment involves consideration of impedance control and cross-talk.
Impedance characteristics are typically determined by transmission line geometry and dielectric properties of the materials in the transmission line circuit. The characteristic impedance of a transmission line circuit is a significant factor in determining the performance of high speed designs. For example, when a signal is reflected back to its source due to a discontinuity caused by an electrical connector or interface in a circuit, such reflections may lead to waveform distortions, which may in turn cause loss in power of the transmitted signal, cross-talk in adjacent lines, and difficulty in transmitting consecutive signals. Cross-talk in a transmission line circuit introduces undesirable signals which cause unpredictable consequences. Cross-talk can be internal resulting from an unwanted signal which may couple from one conductor to another Electromagnetic interference (EMI) may result from electronic noise picked up from an external field. Thus, the characteristic impedance, cross-talk and EMI parameters not only have to be considered in the design of printed circuit boards for desired transmission line signal efficiency, but the electrical connectors in the circuit must also address these parameters.
Present connection systems are frequently used to connect printed circuit boards that are removable. In such systems, a daughterboard may be interconnected through a connector assembly to a motherboard, the daughterboard being replaceable as needed. High pin count connector systems have been developed which locate connection devices, such as plugs or receptacles, for connection to the mother and daughterboards, on relatively close centers, for example 0.100 inches or less in a multi-row matrix so that a large number of circuit interconnects per connector is achieved.
One arrangement of a high density, controlled impedance electrical connector is shown in U.S. Pat. No. 4,917,616 to Demler, Jr., et al. In the device described in this patent, ground planes are dispersed between a plurality of signal pins such that the spacing between the pins and the ground planes is maintained substantially the same. Dielectric material is disposed between the ground planes and the pins, the geometric spacing combined with the value of the dielectric constant of the dielectric material thereby defining the characteristic impedance in a known manner. Other connectors with controlled impedance characteristics are shown, for example, in U.S. Pat. No. 4,881,905 to Demler, Jr., et al and U.S. Pat. No. 4,869,676 to Demler, Jr., et al. In these patents, the controlled impedance is described to be provided by the use of a cast metal housing which places a ground plane equally spaced from the individual signal pins. Other examples of controlled impedance connectors are shown in U.S. Pat. No. 4,836,791 to Grabbe, et al and U.S. Pat. No. 4,762,500 to Dola, et al.
While the known electrical connectors are useful in controlling impedance characteristics and cross-talk parameters, there is a further need to provide higher density pin count in such controlled impedance environments. For example, while known controlled impedance, shielded electrical connectors have pin counts on the order of 40 signal contacts per linear inch of connector, it is desirable to have pin counts on the order of 75-80 signal contacts per linear inch.